Many waveguide designs have been used in an attempt to create a very long-range mode that has low losses while maintaining the ability to have a high signal near a metal surface at a predetermined location. More specifically, various surface plasmon polariton (SPP) waveguides have been developed with different shapes/thicknesses of metal films, including a simple dielectric waveguide structure (a high index layer) on top of or surrounding a metal film to aid in field confinement. However, all of these waveguides retain the simple problem that as the field is concentrated, it is concentrated most at the metal, and hence the losses increase dramatically and the propagation lengths accordingly decrease.
Likewise, waveguides have also been designed to couple received light into a particular waveguide mode (e.g., via a grating). In fact, many studies have been performed that look at grating coupling into SPP modes. However, for these designs, the efficiency is low because the grating meant to couple light into the mode almost as effectively couple light back out of the mode.
A third design consideration in the production of waveguides is in the creation of Bragg mirrors for the mode. More specifically, Bragg mirrors are generally designed to limit the energy lost due to diffraction out of the mode or losses in the mirrors. Gratings have been used with standard SPP modes as Bragg mirrors, but these designs have significant losses due to diffraction.